1. Field of the Invention
This invention relates to digital television apparatus and more particularly, but not exclusively, to digital video tape recorders (DVTRs).
2. Description of the Prior Art
Recently there has been an increasing interest in the use of digital techniques for television signals. In a DVTR, an incoming television signal to be recorded is sampled, the samples are pulse code modulation coded into digital form, the resulting digital data signals are recorded on a magnetic tape and subsequently reproduced, the reproduced digital data signals are decoded, and the decoded signals are used to form an analog signal corresponding to the original television signal.
If errors occur in the handling of the digital signals, for example due to noise or tape drop-out occurring in the DVTR, the digital signals are corrupted and then the reformed analog television signal does not correspond exactly to the original analog television signal, and a resulting television picture is degraded.
There are two main approaches to dealing with errors in digital television signals. The first approach is correction, which involves the production and use of additional data signals purely for the purposes of error detection and correction, these additional data signals otherwise being redundant. While correction provides good results, it cannot generally be used as the sole means of dealing with errors, because a comprehensive correction capability would require an excessive amount of additional data which might overload the data handling paths or raise the data rate to an unacceptable level. The second approach is concealment. This comprises the replacement of corrupted data signals by data signals generated using available uncorrupted data signals. This method relies largely for accuracy on the strong correlation that exists in a television signal.
When a DVTR is reproducing in the normal reproducing mode, that is at the normal replay speed, the data is being recovered from the tape at the same overall rate that it is required for display on the television receiver. It is only necessary to provide time-base correction to synchronize the data reproduced from the tape with stable output synchronizing signals.
Increasingly, however, there is a requirement for reproduction at non-normal speeds. These are generally referred to as special reproduction modes and include still, slow motion, reverse motion and the so-called shuttle mode which comprises the whole range of speeds from fast forward to fast reverse and which is of particular utility in searching and editing. In an analog VTR, slow motion and shuttle speeds are achieved by the repetition or omission of complete fields. In slow motion the VTR uses a few selected fractions of the normal speed, while at the fast shuttle speeds the tracking system of the VTR is unable to reproduce complete fields, and noise bars appear in the reproduced television picture where mis-tracking or cross-tracking occurs.
It is hoped that these restrictions can be reduced in a DVTR, and that the application of correction and concealment techniques will further improve the quality of the reproduced television picture even in special reproduction modes.
Two possible techniques can be adopted to recover data from the tape in a DVTR when the tape moves faster or slower than the normal speed. Firstly, the rotational speed of the drum can be changed in proportion to the change in the tape speed to maintain correct tracking. A serious problem with this is that the data rate also changes proportionally and therefore it would be necessary to accommodate a very large change in the system clock pulse signals. This is difficult to achieve and also implies upper and lower speed limits. Secondly, the rotational speed of the drum can be maintained constant and only the tape speed changed. Inevitably mis-tracking will occur, at least at some speeds, but as much data as possible is recovered. The handling of this reproduced data, relating as it does to parts of fields and containing noise and errors, as well as needing possibly very substantial time-base correction, is troublesome, and it is with these particular problems that the present invention is particularly concerned.